The present invention relates to wireless communication systems and, in particular, to controlling channel congestion in wireless communication systems.
In time division multiple access (TDMA) wireless communication systems based on the well-known IS-136 standard, a set of channels referred to as control channels are used for communicating control information and short messages between the wireless communication systems and mobile-telephones. The control channels typically include at least one reverse channel and one forward channel. The reverse channels and forward channels include a set of reverse sub-channels and forward sub-channels, respectively. The wireless communication systems use the forward sub-channels to transmit short messages, paging messages, etc. to the mobile-telephones. By contrast, the mobile-telephones use the reverse sub-channels to transmit call processing related messages (e.g., call origination and paging response messages), acknowledgments to short messages, etc. to the wireless communication systems. Note that the reverse sub-channels are also referred to herein collectively as Random Access Channels (RACH).
Specifically, the short messages (SM) are transmitted by the wireless communication systems to a particular mobile-telephone on a set of forward sub-channels referred to herein as Short Messaging Service Channels (SMSCH). A short message can be one of two types: (1) an informative message, such as a numeric page; or (2) a request for the mobile-telephone to respond with an acknowledgment. If the short message is a request, the mobile-telephone can respond to the wireless communication system with a SM acknowledgment on the RACH. Typically, the ratio of SM requests to SM acknowledgments is one-to-one.
Broadcast Tele-services, as currently proposed for IS-136, will allow the wireless communication systems to transmit to every mobile-telephone within the coverage area of the wireless communication system, or part thereof, a broadcast short message (BSM) on a set of forward sub-channels referred herein as Broadcast Channels (BCCH). Similar to a short message, a broadcast short message can be one of two types: (1) an informative message, such as stock quotes, sports news, etc.; or (2) a request for each of the mobile-telephones to respond with an acknowledgment, such as a vote. If the broadcast short message is a request, the mobile-telephone can respond with a BSM acknowledgment on the RACH. Unlike SM requests, the ratio of BSM requests to BSM acknowledgments is one-to-many. Thus, if a large number of mobile-telephones are attempting to transmit BSM acknowledgments at the same time over the same RACH, collisions are likely to be caused among the acknowledging mobile-telephones (i.e., mobile-telephones attempting to send a BMS acknowledgment), and between the acknowledging mobile-telephones and mobile-telephones transmitting call processing related messages over the same RACH. In other words, the RACH may become congested as a result of numerous mobile-telephones attempting to transmit on the RACH at the same time. Such congestion prevents call processing related messages and BMS acknowledgments from being successfully received by the wireless communication system. Accordingly, there exists a need to control random access channel congestion that result from a large number of mobile-telephones transmitting acknowledgments to broadcast short message.
One embodiment of the present invention is a method for controlling random access channel congestion that result from a large number of mobile-telephones transmitting acknowledgments to broadcast short messages. In this embodiment, BSM acknowledgment transmissions on the random access channel are controlled by the wireless communication system using random access channel restrictions and random access techniques. Random access channel restrictions are used to reduce collisions among acknowledging mobile-telephones and call processing mobile-telephones by limiting the BSM acknowledgment transmissions to particular random access channels. Random access techniques are used to reduce collisions among acknowledging mobile-telephones by distributing the BSM acknowledgment transmissions over time.
In another embodiment of the present invention, the wireless communication system transmits a broadcast short message with a bit map and a time delay parameter. The bit map indicates which particular random access channels may be used for transmitting BSM acknowledgments. The time delay parameter is used by the mobile-telephone in conjunction with the output of a random number generator to determine the duration of a random delay period. Upon the end of the random delay period, each of the mobile-telephones will attempt to transmit an acknowledgment to the broadcast short message using the random access channels that were indicated in the bit map.